The invention concerns an apparatus and a process for determining the relative position of a tool and a workpiece prepared for machining on a machine tool.
In the machining of workpieces, the exact point of machining contact of the tool is in many cases not visible because the tool itself hides the point of machining. This applies for example to the continuous generating grinding of gear teeth, particularly in the case of helical gears. On account of the particular form of the tool employed in this process, i.e. the grinding worm, and the conditions of contact between it and the gear teeth to be machined, in which the working positions are only derived by mathematical consideration (intersection), it is not possible to detect the exact point of machining contact, i.e. the point of mesh on the workpiece, optically. Hence when setting the machine the position of the tool and workpiece one to the other, which is required for fixing the starting and end positions of the machining process, cannot be determined by direct visual means.
When setting the machine for machining such a workpiece, the exact axial position of the latter relative to the work spindle after setting up is not usually known, as it is dependent on many different individual dimensions of the work driver, work arbor, spacer collars, and of the workpiece itself. This likewise eliminates the ability to have the axial position of the gear teeth to be machined, relative to the point of machining contact of the grinding worm, calculated by the numerical machine control system by way of the position of the work spindle, although the position of the latter relative to the machine coordinates is just as well known as the point of machining contact of the tool, i.e. the tool reference point. Especially when operating with tapered work arbors the axial workpiece position differs with every set-up, as it is dependent on the bore tolerance or allowances.
In practice one resorts in such cases to the use of mechanical devices such as measuring indicators or the like, which represent in some manner the height of the tool reference point, in order to visually determine the axial position of the workpiece or its teeth. This method is unfavourable as, apart from the the limited attainable accuracy, the differing workpiece sizes often also make differing measuring indicators necessary. Moreover the measuring indicators must be removed from the working area again when the exact axial position of the workpiece has been determined.
A suggestion has also already been made to measure the axial position of the workpiece teeth to be machined relative to the work spindle or to the machine coordinate system by means of a length gauge, and to subsequently put the determined dimension into calculation with the actual position of the tool reference point. But this method also demands, as does the measuring indicator mentioned, a reference surface in the machining space, which is on the one hand difficult to realize and on the other hand means inconvenient manipulations in the generally oily machining space.
The object of the present invention is to provide a device and a process for determining the axial position of the workpiece teeth on the machine with the workpiece set up, without manual manipulation in the machining space, which is equally suitable without adaptation for all workpiece diameters in question. This task is fulfilled by the combination of features in accordance with the claims.
By means of a laser line projector, which is located in the working area of the machine such that the beam plane it produces is at right angles to the workpiece axis and contains the tool reference point, or is at a known distance from the latter, a line of light is projected onto the circumference surface of the workpiece to be measured. If this beam plane is moved relative to the workpiece in the workpiece axial direction, the projected light line also moves across the gear facewidth. If, with reference to the workpiece, the starting position for the machining programme has been defined for example as the upper end face of the workpiece teeth, then the starting position is attained as soon as the laser light line coincides with the upper end face of the teeth. If the laser line projector is so located that the laser beam plane does not contain the tool reference point, then after attaining the coincidence position the distance between the tool reference point and the laser beam plane must be compensated by an additional axial workpiece displacement corresponding with this distance.